Wireless soil profile monitoring apparatus and methods

ABSTRACT

An in situ ultra-low power contactless measurement apparatus and method suitable for micro-electronics in big data applications for continuously reporting a soil moisture profile at various zones.

FIELD OF THE INVENTION

The invention relates generally to measurement devices and morespecifically to measurement devices for measuring characteristics of asoil profile including volumetric water content, electrical conductivityand temperature at various depths in the soil.

BACKGROUND OF THE INVENTION

The present invention is primarily focused on taking accuratemeasurements of volumetric water content (VWC) in a variety of media,including but not limited to: soil, wood, concrete, building materials,clothing, and the like. This invention is useful in such applications ascrop irrigation, lawn care, gardening and other landscapingapplications, laundry systems, non-destructive testing for ground waterinfiltration into buildings, and a full range of moisture environmentsranging from dry to completely saturated.

The present invention is particularly suited for big data environmentssuch as large scale deployment of numerous sensors such as industrialirrigation systems and municipal irrigation systems because the presentinvention addresses the issues of accuracy and total lifecycle cost ofsoil monitoring devices. The incident methods are intended for but notlimited to synthesis into microchips.

In relation to capturing a set of soil vital signs in a big dataenvironment there is particular interest on the accurate measurement ofvolumetric water content in soil at large numbers of zones. However,this invention has wider applicability.

The present invention sets forth a low-cost, ultra-low power moisturesensor and integrated communication system to make remote wirelessapplications possible which utilize continuous unattended monitoring ofa multiplicity of locations or zones. Further, the present inventionprovides for the implementation of inexpensive sensors with improvedlongevity, durability, security, reliability and wide spreadapplicability.

Measuring soil moisture content at a municipal or industrial scale iscomplex and includes a number of well-known problems in order to measureto an adequate level of accuracy and reproducibility at a reasonablecost. Often at the municipal and industrial level, deployment of a largenumber of sensors impacts the overall system cost because of therequirement to frequently replace batteries and the need to dean sensorprobes to ensure adequate accuracy. In addition, the large number ofsensors must work in a wide range of soil types and soil conditions fromdry to fully saturated.

There are a number of known techniques for measuring soil water contentincluding (a) Neutron probe—uses radioactive material which is expensiveand is typically inaccurate in topsoil (b) Matric potential—uses lowcost gypsum which has slow response and lacks durability (c)Tensiometers—require regular maintenance (d) Time DomainReflectometer—is expensive and is only accurate up to 65% VWC (e)Capacitive—is low cost but is susceptible to electrical conductivityissues (f) Frequency—low cost but is susceptible to electricalconductivity issues and is limited in range (g) ImpedanceMatching—expensive and limited range.

The following references are representative of some of the known devicesand techniques for measuring soil water content.

U.S. Pat. No. 7,944,220, teaches the performance of a dielectricmoisture content sensor which is commonly limited by sensitivity tosalinity and nutrient levels in the soil, as well as sensitivity totemperature change. This is in part due to soil non-homogeneity andvariation of soil composition. Most crops are grown in soil with asalinity and nutrient level corresponding to an electrical conductivitybetween 60 mS/m to 400 mS/m and as high as 500 mS/m to 600 mS/m forcertain crops, such as tomatoes, with soil conductivity in coastalenvironments up to 3,000 mS/m. Temperature also creates large variationsand must be considered in any viable method. Salinity and othernutrients are the primary cause of significant deviation and must beconsidered. The result of these issues it that most reasonably pricedsolutions do not function or measure VWC above 65% water saturation.

U.S. Pat. No. 5,424,649, is representative of a common technique used inlow cost sensors that have a thin dielectric coating. The dielectriccoating is only partially effective at reducing sensitivity to soilconductivity which results in a moisture content sensor with sensitivityto soil conductivity and salinity. Coatings are also subject to wear anddo not address the issue of charge stealing by the earth.

U.S. Pat. No. 5,859,536 is representative of common techniques in morecostly sensors which use impedance matching networks. Because thesetechniques depend on current flow into the earth, it is naturallysusceptible to conductance of soil to ambient ground.

U.S. Pat. No. 5,804,976 is a more reliable technique utilizing atransmission line and measuring propagation delay. However, thistechnique also suffers near total loss of signal at high saturation.

U.S. Pat. No. 7,030,630 is a moisture sensor with a capacitive moisturemeasuring element and method of determining air humidity describes timeconstants associated with a parallel resistance but is silent regardingthe parasitic series resistance.

Current models and consequently the methods currently in use areeffective for measuring humidity as in U.S. Pat. No. 7,030,630 but donot function in situ of greater that 65% volumetric water contentmaterial.

U.S. Pat. No. 5,730,165 teaches that if the sensor employs an RC circuitor variation thereof, the stray conduction path will rob the plate ofcharging current and will thus alter its apparent time constant.

Studies including the IEEE Experimental Electrical Modeling of Soil forIn Situ Soil Moisture Measurement 2013 are instructive and provide amore accurate electrical model for soil over the entire range ofsaturation. This model, however, must be adapted to accurately representthe electrical parameters of sensors embedded in the earth.

Often the previously known sensors require battery replacement eachseason which is a significant limitation to large scale deployment.

Cost of the sensors and vulnerability of current sensors to tamperingfurther prohibits wide deployment in unsecured areas.

As a result, all of the current techniques have proven uses inparticular segments but they suffer at least in terms of cost,complexity or accuracy, thus preventing ubiquitous adoption for largescale data collection such as in a metropolitan or industrial sensornetwork.

SUMMARY OF THE INVENTION

The present invention is a volumetric water content sensor comprising anintegrator circuit, switched capacitors and conductors placed inproximity of the material to be sampled.

The signal applied to the sample is transient with very high frequencycomponents.

During each measurement only a portion of the transient response iscaptured by the integrator.

Additionally, at least two separate measurements may be used incombination to determine volumetric water content and electricalconductivity.

In this case, each measurement consists of generating a control signalwith fixed period and different fixed duty cycle for a fixed number ofcycles.

During each measurement a different portion of the transient response iscaptured by the integrator making it possible to extract VWC andelectrical conductivity (EC).

One aspect of the invention is accounting for charge stealing by theearth in highly saturated soil. Considering in situ measurement using athree terminal model instead of two allows accurate measurement of 0% to100% VWC media and true separation of the measurements of thepermittivity and conductivity of the media, whereas conventional sensorscease to operate above 65% MC.

A second aspect of the invention is measuring permittivity and EC by twomeasurements including a long and short sample window. A sample windowis formed by timing the diverting of the electrical current between twoseparate sampling capacitors during successive charge events. A firstmeasurement is made using a short window and second measurement is madeusing a long window. The window time is the percentage of time the firstcapacitor is charged compared to the charge event.

This allows for a non-galvanic connection to the media which preventsoxidation issues due to exposed metal. It also prevents electroplatingissues, thus extending the life of the sensor. Unlike traditionalsensors it provides a full range measurement including dry to completelysaturated media.

A third aspect of the invention is aggregating thousands of individualmeasurement events per second in the analog hardware. This allows muchgreater sensitivity in completely dry son. It also provides accuracy asa tradeoff with time. The incident method typically provides 12 bits ofaccuracy in less than 2 milliseconds.

A fourth aspect of the invention is the use of a transient measurementtechnique instead of an AC signal. This allows very high frequencies tobe used resulting in measurements taken 100 times faster thantraditional methods.

The components, in contrast to traditional methods, do not includeinductors or expensive oscillators and are suitable for synthesis intoinexpensive microelectronics.

A fifth aspect of the invention is that of detecting tamper bydetermining if the sensor has been unexpectedly moved or disturbed by,for example, a person or animal.

The sensor may be housed in a durable waterproof enclosure and embeddedin the earth or remotely mounted to a permanent structure such as afence post or plant trellis.

An example deployment is to directly burying the sensor in irrigationpipe trenches with antenna cable extending to sprinkler risers.

The plurality of probes can be configured to include any number of zoneswith individual volumes of influence, including profiling the soil foradequate drainage and migration of nutrients.

Comparisons with reference material inside the sensor removes the needfor soil standards or recalibration.

The low output impedance and low input impedance provided by this methodprovides an important mechanism for placement of the sensor electronicsup to several meters from the probes by providing shielding up to thevolume of influence for each of several zones. The external antennaconnection provides further reach to above ground structures and putsthe sensor out of sight.

Because RF components are not used there is an associated reduction inthe need for power. The reduced power consumption of the presentapplication can provide four (4) updates per day for over six (6) yearson AA batteries making it suitable for long term deployment; thus makingit maintenance free for the entire lifecycle of the sensor. Whencombined with low cost metropolitan and industrial network radiomicrochips such as LoRaWan™ and SigFox™, it is suitable for public bigdata applications including thousands of zones over greater than a 15 Kmradius and up to 1 Km in urban environments.

The sensor may be configured to measure the moisture content of soil.However, the moisture content sensor may be arranged to measure othermediums such as wood, concrete, textiles, moisture sensitive polymers,etc.

The probes may be constructed of nearly any conductive or semiconductive material and may be coated or left in contact with the mediaand include openings for moisture to pass freely.

An embodiment of the apparatus and method may include the moisturecontent/EC sensor as well as a temperature sensor to measure thetemperature of the media and surrounding media, humidity sensor, rainfall sensor, light sensor, wind sensor and the like for the predictionof future change in moisture.

The invention stores reference profile data for the purpose of detectingremoval or tampering of the sensor. It also includes cryptographic keysfor preventing eavesdropping or man-in-the-middle attack, making itsuitable for industrial and municipal applications.

The invention is advantageous for tracking the factors for, andprediction of future moisture content in the media, thus enabling morejudicious use of water and when coupled with precipitation forecastinformation it provides the platform for ubiquitous large datacollection and efficient irrigation and prevention of water damage.

In one embodiment, vertical arrangement of up to seven zones track thepropagation of moisture in the media. This is advantageous to accountfor hysteresis and allow for creating critically damped volumetric watercontent in the media and prevent runoff or starvation of water. It alsoprovides a mechanism for tracking the propagation of nutrient fertilizerthrough the soil. This information is used to optimize watering patternsand fertilizer patterns for various soil types and prevent undesirablerunoff of bio-nutrients, which is a known cause of algae outbreaks inmany lakes.

Measurement of permittivity and electrical conductivity of the media ismade by successive long and short window measurements either bycapturing rising edges with a short window and falling edges with a longwindow or by performing a series of short window events followed by aseries of long window events, or any combination thereof. This allowsthe sensor to function in completely dry media or when completelysubmersed in water.

The measurement system is autonomous from the CPU. It is configured toexecute N number of events then perform the analog to digital conversionand then activate the CPU. The CPU successively schedules long and shortwindow measurements. It also schedules the other sensors such astemperature, light, humidity. After all of the data is collected, theCPU then activates the radio local oscillator and sends the data to theInternet repository, then notifies the power management timer to poweroff everything for for a period of time, six hours.

A rapid measurement using only a very small number of events isperformed periodically to detect tamper and perform rapid notificationby storing and comparing against stored previous measurements.

The features discussed in relation to any of the aspects of theinvention may be applied to any other of the aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a first embodiment with vertical profile arrangement of zones.

FIG. 2 is a second embodiment with an alternative zone of influence.

FIG. 3 is a surface profile arrangement of zones.

FIG. 4 is a distributed zone arrangement.

FIG. 5 is a schematic of a moisture sensor system.

FIG. 6 is a graph containing example measurement data.

FIG. 7 is an accuracy comparison of example measurements.

DETAILED DESCRIPTION OF THE INVENTION

First Embodiment

FIG. 1 shows a volumetric water content sensor comprising an integratorcircuit for measuring soil moisture content 1. The VWC sensor measuresmoisture content at seven zones within the soil with each zone having adistinct volume of influence 19. A volume of influence is the spacewithin the soil in electrical proximity of probe 17 and probe 18. Theapparatus shown in FIG. 1 also includes a radio and radio antenna 10.The radio provides direct connection and logging of sensor data to a bigdata server via network including the Internet, WiFi, WiFi Max,LoRaWan™. The apparatus further includes a plurality of sensors formeasuring other parameters which may be used in combination with thewater content measurement for predicting soil conditions. The pluralityof sensors may include: a topsoil temperature sensor 15, a roottemperature sensor 16 and an air temperature sensor 13, a humiditysensor 12 and a light sensor 11. The electronics for the moisture sensorare housed in an enclosure 14. The enclosure is configured to be waterproof.

The electronics in this embodiment are housed with or adjacent to themoisture probes referring to Zone 1 Probe A denoted by 17 and Zone 1Probe B denoted by 18. Each set of probes placed adjacent or near eachother form a continuous profile of moisture volumes of influence in theearth 20 over a certain depth. It should be noted that the probes mayalso be adjacent to the material being sampled and that the material maybe any type of permeable media.

Each zone is comprised of two probes. Multiple zones are arrangedtogether to measure a VWC and EC profile at their respective depths orlocations in the medium.

Second Embodiment

The second embodiment shown in FIG. 2 includes the means for measuringsoil moisture content also at several levels within the soil andincludes the plurality of sensors. The probes in this embodiment,however, are spaced horizontally and provide for a larger volume ofinfluence by further separating the probes. Volumes of greater than acubic meter can be sampled using the method of this invention. It isnoted that previously available sensors measure up to only a fewcentimeters.

Third Embodiment

FIG. 3 illustrates a third embodiment which includes a means formeasuring topsoil moisture content at numerous locations. The probeconductors in this embodiment are spaced around a wheel such as on thewheel of a tractor or lawn mower. Samples may be correlated with GPScoordinates to create a detailed topsoil moisture map.

Unlike traditional methods of moving a sensor above the surface ordragging a sensor across the material to be sampled, the currentinvention provides a consistent amount of pressure and contact with thesoil.

The current apparatus is useful to reduce the issues associate with airgaps between the soil and sensor probes. The current invention alsoapplies to radio frequency backscatter measurement of volumetric watercontent as well as time domain reflectometry measurement of volumetricwater content.

Fourth Embodiment

FIG. 4 illustrates a third embodiment which includes a coaxial cable 35connection between the various probes 36 and the enclosure 14 encasingthe electronics. The driver shield 32 of the coax greatly reduces theparasitic capacitance of the coaxial cable 35. Likewise the antenna 10may be separated from the enclosure 14 using the antenna coax cable 31.This allows placement of the probes under ground level in a high trafficarea and hiding the electronics from view under the ground and allowingthe antenna to be housed above or at the surface of the ground in aseparate casing.

The embodiments are to illustrate a few of the many possibleconfigurations for measuring soil moisture content. It is also possible,for example, to define the zones to cover a physical volume, for examplein a “square foot gardening” grow box or in separate plant containers orseparate crop rows or to provide redundancy for accuracy.

Method

FIG. 5 illustrates an electrical schematic of the moisture sensor systemwhich provides the means of measuring soil moisture content. Theelectronics includes the following components: 41 CPU or radio System OnIntegrated Chip (SoIC); 42 Embedded Sensor Driver module; 44 Integrator;45 first bank or switches or Quad DPQT CMOS Switch, 46 second bank ofswitches or Dual DPQT CMOS Switch; 47 Reference Impedance; 410 (411-416)Bank of Capacitors, 421-423 Voltage References.

Control signals for the soil volumetric water content saturation areprovided by Embedded Sensor Driver Module 42 of the CPU 41. The EmbeddedSensor Driver Module 42 outputs two (2) Pulse Width Modulated (PWM)signals used to control the first bank of switches 45 associated withcapacitor bank 410 (411, 412, 413, 414), and the second back of switches46 associated with capacitor bank 410 (415, 416). The Embedded SensorDriver Module 42 also includes an Analog to Digital Converter (ADC) tocapture the output of the Integrator 44.

The measurement sequence begins by the CPU 41, creating two outputsignals, for example, a period of 4.4 microseconds. A first measurement,M01, is made using control signal 445, for example, at 25 percent dutyand control signal 446, for example, with 50 percent duty cycle.

The phase difference between signal 445 and signal 446 is made to beoverlapping with signal 445 leading signal 446, for example, a 0.3microseconds overlap. This creates a sequence of alternating phase I andphase II cycles.

During Phase I: Capacitor 411, Capacitor 415, Capacitor 412 and theprobe are placed respectively in series with each other. Capacitor 416is placed across ground Reference 423, Capacitor 413 is placed acrossthe +5 Volt Reference 421 and −5 Volt Reference 422, and Capacitor 414is conversely placed across −5 Volt Reference 422 and +5 Volt Reference421.

During Phase II: Capacitor 413, Capacitor 416, Capacitor 414 and theprobe are respectively placed in series with each other. Capacitor 415is placed across the integrator and Ground Reference 423, Capacitor 411is placed across +5 Volt Reference 421 and −5 Volt Reference 422, andCapacitor 412 is placed across −5 Volt Reference 422 and +5 VoltReference 421.

The alternating sequence produces an alternating 20 Volt signal toappear across the probe terminals 17 and 18.

In this case, during Phase II the integrator sums the current collectedin the probe during Phase I. This allows for a very short window forcollecting the current in the probe during Phase I and a long time forthe integration during Phase II. This allows for an inexpensive OpAmp,for example, <1 MHz Gain Bandwidth. The resulting filter responsehowever is capable of capturing frequency components >5 MHz for vastlyimproved bound moisture detection.

The CPU 41 continues this sequence for a specified number of cycles, forexample, 0.25 milliseconds to allow the probe to reach a nominal averagecommon mode voltage.

The CPU 41 then releases the Reset 447 on the Integrator 44.

The CPU 41 continues this sequence for another specified orpredetermined number of cycles, for example, a total of 256 cycles or1.75 milliseconds longer. This produces a full range of 0 Volts for drymaterial to 2.5 Volts for 100% saturated material with very highprecision in under 2 ms. For incredibly precise measurements or forextremely large volume of influence the number of cycles can beincreased to approximately 5000 cycles limited only by the inputreferred offset of the OpAmp.

The CPU 41 then stops both control signal 45 and control signal 446.

The CPU 41 then starts an Analog to Digital Conversion, ADC.

The CPU 41 records the ADC measurement as M01.

The CPU 41 then repeats the measurement sequence with control signal 445and control signal 446 both at 50 percent duty cycle.

The CPU 41 then records the ADC measurement as M02.

The CPU 41 then signals the switch 46 to connect the probe terminals 17and 18 to the Reference Impedance 47.

The CPU 41 then repeats the measurement sequence with signal 445 at 25percent duty cycle and records the ADC measurement as M11.

The CPU 41 then repeats the measurement sequence with signal 445 at 50percent duty cycle and records the ADC measurement as M12.

The CPU 41 then signals the switch 446 to connect the probe terminals toZone(N) and repeat the same measurement sequence to record MN1 thoughMN2 for each zone.

The CPU 41 then computes the raw saturation using the following steps.

The measurements MN1 and MN2 are adjusted to remove the parasiticcapacitance by subtracting M01 and M02, respectively.

The measurements are then scaled to account for temperature and supplyvoltage variation, resulting in Mz1 and Mz2, e.g., by multiplying by thescaling factor of M11 and M12 from the 50 percent reference 47.

Saturation for each zone is then extracted based on the ratio of theduty cycles. For example with 25% and 50% using the formula,Saturation=(2*Mz1−Mz2)*100%.

Electrical Conductivity for each zone is then extracted using theformula EC=(2*Mz2−Mz1)

Improved accuracy can be achieved by adjustments to the formula ratio asnecessary based on actual measured window sizes for individual apparatusembodiments. Generally, simply using the control signal duty cycle ratioproduces better than 2% accuracy at nominal temperatures.

The sequence described is only one specific representation of the methodof using different window sizes to extract saturation and electricalconductivity. The method applies more generally to using differentwindow sizes.

For example a single capacitor may be successively applied to the probeterminals and the probe terminals being successively shorted togetherwhile the capacitor is placed across the integrator configured to areference voltage. Likewise, a pair of capacitors may be successivelyapplied to the probe and an integrator. In each case at least twodifferent measurements are made using different window sizes.

Regarding wireless soil profile measurements being controlled by a RadioSystem on Integrated Chip (CPU). The Radio CPU wakes up periodicallysuch as four (4) times a day and performs the tasks of makingmeasurements from the various sensors typically by issuing commands viaan I2C bus. The Radio CPU also provides the necessary functions formaking a moisture measurement. Once it has collected the soil profiledata including temperature, humidity, light level, saturation, andelectrical conductivity it powers up the radio module and sends the datavia LoRaWan™, SigFox™, WiFi or other modulation scheme to an Internetbased big data cloud service or private database. The service combinesthe data with other sensor data and satellite and forecast data andapplies heuristics to the data and issues updates and demands forirrigation.

EXAMPLE

Measurements were taken on soil samples of white clay with variousamounts of water content using the example values presented above.

Saturation is defined as the ratio of volume of water to the totalvolume of space or voids in the soil. The samples were independentlymeasured using density of water 1 g/cc and the density of air driedwhite clay 0.97 g/cc to have the Saturations of 7%, 25%, 37%, 38%, 55%,72%, and 89%.

Examination of FIG. 6 shows the ADC measurement values of each of thesamples electrical models using two different window sizes; M1 of 0.55microseconds and M2 of 2.0 microseconds. The measurements were takenwith a duration of 2 milliseconds using a 20 Volt peak to peak stimuluswith period of 4.4 microseconds.

Extraction of Saturation was accomplished using the formulaSaturation=2M1−M2. The calibration measurement of zone “None” is used tooffset parasitic capacitance of the apparatus. Zone “Ref” is used to mapthe Saturation to Percent Volumetric Water Content. This formula isbased generally on the windows sizes of 0.55 microseconds and 2.0microseconds. The equation used may be adapted to address specificwindow sizes and other patristics of the apparatus or environment asnecessary such as when in containers or varying temperature and thelike.

FIG. 7 shows the Measured Saturation vs the Gravimetric ComputedSaturation in this example. The results agree within 2% over the entirerange of very dry to completely saturated white clay.

Although the invention has been described relative to a specificembodiment thereof, there are numerous variations and modifications thatwill be readily apparent to those skilled in the art in light of theabove teachings. It is therefore to be understood that, within the scopeof the appended claims, the invention may be practiced other than asspecifically described.

The invention claimed is:
 1. A method for measuring characteristics of amaterial by an apparatus, comprising: performing a set of measurementsequences, wherein each measurement sequence comprises: generating, fora plurality of cycles, a first control signal and a second controlsignal, wherein the first control signal and the second control signalhave a same period length; applying, for the plurality of cycles, thefirst control signal to a first switch bank that is coupled to a firstcapacitor bank, and the second control signal to a second switch bankthat is coupled to a second capacitor bank; and aggregating, by anintegrator for the plurality of cycles, a current of a capacitor of thesecond capacitor bank, wherein the current is collected when thecapacitor is coupled in series with a probe; measuring, for the set ofmeasurement sequences, an output of the integrator to produce a set ofmeasurements, wherein the set of measurements comprises a firstmeasurement taken with the first control signal having a first dutycycle and a second measurement taken with a second duty cycle that isdifferent from the first duty cycle; determining a saturation based onthe first measurement, the second measurement, and a ratio of the firstduty cycle and the second duty cycle; and transmitting data indicatingthe saturation.
 2. The method of claim 1, wherein each measurement cyclecomprises: creating, by a processor, a first phase alternating with asecond phase based on the first control signal and the second controlsignal; switching, by the first switch bank, a first capacitor, a secondcapacitor, a third capacitor, and a fourth capacitor of the firstcapacitor bank based on the first control signal, and switching, by thesecond switch bank, the capacitor and a sixth capacitor of the secondcapacitor bank based on the second control signal, wherein: during thefirst phase, the first capacitor, the capacitor, the second capacitor,and the probe are placed in series, the sixth capacitor is placed acrossa ground reference, the third capacitor is placed across a firstreference and a second reference, and the fourth capacitor is placedacross the second reference and the first reference; and during thesecond phase, the third capacitor, the sixth capacitor, the fourthcapacitor, and the probe are placed in series, the capacitor is placedacross the integrator and the ground reference, the first capacitor isplaced across the first reference and the second reference, and thesecond capacitor is placed across the second reference and the firstreference, and the integrator sums a current collected in the probeduring the first phase.
 3. The method of claim 2, wherein the firstreference provides +5 volts and the second reference provides −5 volts.4. The method of claim 1, wherein the material is soil.
 5. The method ofclaim 1, wherein the period length is 4.4 microseconds for each of themeasurement sequences.
 6. The method of claim 1, wherein transmittingthe data comprises communicating, by wireless transmission, the dataindicating the saturation to a server or database.
 7. The method ofclaim 1, wherein each of the plurality of cycles produces an alternating20 volt signal across terminals of the probe.
 8. The method of claim 1,wherein the first control signal has a 25% duty cycle and the secondcontrol signal has a 50% duty cycle for a first measurement sequence,and wherein the first control signal has a 50% duty cycle and the secondcontrol signal has a 50% duty cycle for a second measurement sequence.